The present invention relates generally to a novel method of electromagnetic geophysical exploration to obtain a representation of the earth's resistivity structure.
In geophysical exploration, several techniques have been developed to ascertain the nature of the earth's subterranean formations. Of these techniques, seismic exploration is the most commonly used, while magnetotelluric, magnetic, and gravity exploration have also proven to be advantageous in selected situations. The traditional approach of geophysical exploration in virgin areas has been to employ gravimetric and magnetic surveys, as well as magnetotelluric surveys, if the earth's subterranean formations have significant differences in electrical conductivity.
In particular, the magnetotelluric geophysical exploration technique obtains a measure of horizontal components of the earth's electrical field and horizontal components of the earth's magnetic field induced in the earth by naturally-occurring fluctuations caused by such things as solar winds, solar flares, diurnal effects, etc. In magnetotelluric exploration, one simultaneously measures both the horizontal components of the electric field and the horizontal components of the magnetic field at a plurality of sites on the earth's surface. The measured horizontal components of the electric field E(.omega.) and magnetic field H(.omega.) are linearly related according to: EQU E(.omega.)=Z(.omega.).multidot.H(.omega.) (1)
where Z(.omega.) is the magnetotelluric impedance tensor at the measurement site and .omega. is the angular frequency of the natural electric field E(.omega.) fluctuation. In practice, the electric field E(.omega.) and the magnetic field H(.omega.) are determined by Fourier transformation of the measured time series E(t) and H(t) The impedance tensor Z(.omega.) is determined by the distribution of resistivities in the earth's subsurface. If the impedance tensor Z(.omega.) is determined at a sufficiently large number of sites in an exploration region, it is possible to uniquely determine the earth's subsurface resistivity distribution. The frequency .omega. of the detected electric E(.omega.) and magnetic H(.omega.) fields provides an indication of the depth to which the resistivity structure influences the impedance tensor Z(.omega.). Generally the depth of investigation increases with decreasing frequency .omega.. In practice, the requisite number of impedance tensor Z(.omega.) measurements needed for a unique interpretation is rarely achieved and this leads to serious ambiguities in interpretation of magnetotelluric data.
By way of example, the earth can be considered to have a layered resistivity structure which varies with depth except for a small, near-surface anomaly r.sub.4 in proximity to measurement site A as depicted in FIG. 1. The resistivity r of the various layers are r.sub.1 .noteq.r.sub.2 .noteq.r.sub.3 .noteq.r.sub.4. The near-surface anomaly r.sub.4 is of no exploration interest in itself and the spacing between measurement sites as well as the range of frequencies .omega. to be measured are determined by the exploration depths which are of interest. Ideally, electromagnetic measurements made at two measurement sites A and B of FIG. 1 would determine the same basic depth varying resistivity structure. However, in FIG. 1, the electric field E(.omega.) measurements made at site A will differ substantially from those made at site B due to the presence of the near-surface anomaly r.sub.4. This difference will persist over the whole range of measurement frequencies. The effect of the near-surface anomaly r.sub.4 is principally to distort the electric field E(.omega.) around it; the magnetic field H(.omega.) is not seriously affected by such near-surface anomalies. Such effects are commonly called "statics" and come into play any time near-surface current flow is significantly controlled by electrostatic electric fields.
The distortion in the measured electric field E(.omega.) caused by near-surface anomalies can result in serious misinterpretation of the earth's resistivity structure using the data from site A. To ameliorate this problem with magnetotelluric measurements, it is necessary to increase the upper limit of measured frequencies .omega. and the number of measurement sites so that the structure of the near-surface anomaly can be identified and its effects removed. This remedy can be very costly, especially considering that the near-surface anomaly is of no intrinsic exploration interest. Warner U.S. Pat. No. 4,473,800 also discloses a magnetotelluric technique for resolving such near-surface anomalies.
Attempts have been made by others to obtain a measure of the earth's resistivity structure independent of electric field measurements which can be extremely sensitive to near-surface anomalies. Specifically, A. F. Kuckes in "Relations between Electrical Conductivity of a Mantle and Fluctuating Magnetic Fields," Geophysical J. R. astr. Soc. 32, p. 119-131 (1973) describes a technique wherein the in-phase and out-of-phase ratios of the vertical component of the magnetic field to a horizontal magnetic field gradient can be used to obtain a measure of a one-dimensional earth resistivity structure.
Additionally, F. E. M. Lilley, et al., in "On Estimating Electrical Conductivity Using Gradient Data from Magnetometer Arrays," J. Geomag. Geoelectr., 28, p. 321-328 (1976) refines the one-dimensional technique of Kuckes to reduce errors generated by locally-induced anomalous fields. Such one-dimensional techniques employing both the vertical component of the magnetic field and horizontal magnetic field gradients are generally unsatisfactory and incapable of adequately describing a two or three-dimensional earth resistivity structure.
More recently, A. D. Richmond, et al., in, "Three-Dimensional Analysis of Magnetometer Array Data," Journal of Geophysics, 54, p. 138-156 (1983) describes a technique for mapping magnetic field variations using data from an array of magnetometers based on optimal linear estimation. This method provides statistical estimates of a spatial auto-correlation function of magnetic field variations from which earth resistivity can be obtained.
To avoid problems associated with electric field distortions associated with magnetotelluric techniques, and to provide a more straightforward method for evaluating one-, two- and three-dimensional earth resistivity structures, the present invention discloses a technique for interpreting multidimensional earth resistivity structures based only on measurements of the magnetic field H(.omega.). The magnetic field H(.omega.) is determined by the spatial distribution of electric currents in the earth. At high frequencies .omega., when current flows at shallow depths in the earth, a near-surface anomaly can affect the magnetic field H(.omega.). However, at lower frequencies, corresponding to depths of exploration interest, the bulk of induced current flow is deep in the earth, and only a small fraction of the total current flow is distorted by the anomaly. As a result, magnetic field H(.omega.) measurements become decreasingly sensitive to near-surface anomalies as the frequency .omega. is reduced. This can be contrasted with the persistent distortion caused in the electric fields E(.omega.) for all frequencies.